The present invention relates to an adhesive composition and an optical device constructed by bonding together optical parts with an optically transparent adhesive layer formed therefrom.
Bonding technologies used for the assembly of optical parts and optical elements used in optical fiber communication systems must have high reliability. For the assembly of optical parts, soldering, laser welding and organic adhesives such as acrylic and epoxy resins have been used. A refractive index control precision adhesive is disclosed by (i) JP-A 6-073358 and JP-A 6-073359 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d), and an organic-inorganic adhesive prepared by a sold-gel method is described in (ii) Journal of Non-Crystalline Solids, vol. 80, pp. 557-563, 1986 and (iii) Intl. Congr. On Glass, pp. 429 to 436, 1986. An optical element assembled by using a sold-gel adhesive comprising an alkoxide and a metal salt is disclosed by (iv) Japanese Patent No. 1829914 (JP-A 62-297369) and a prism assembled by using an adhesive comprising a silicic acid salt and an alkoxide is disclosed by (v) Japanese Patent No. 2786996 (JP-A 7-5307). Further, an organic-inorganic composite adhesive is disclosed by (vi) USP 5991493.
(vii) JP-A 11-343474 discloses an adhesive composition comprising an epoxy resin, organoalkoxysilane and aluminum chelating compound. (viii) JP-A 9-243870 teaches that an optical part is sealed by an optically curable resin composition material which comprises a bisphenol type epoxy resin, novolak type epoxy resin, optically acid generating agent and epoxysilane as a coupling agent. Further, (ix) JP-A 2000-109780 discloses an ultraviolet curable adhesive composition for optical parts which comprises a bisphenol A type epoxy resin, alicyclic epoxy resin, optical cationic initiator and xcex3-glycidoxypropyl trimethoxysilane.
However, the above bonding technologies and adhesives for optical parts have the following problems. The soldering and laser welding are unsatisfactory in terms of fixing position accuracy and need a laser light source and advanced technologies. The epoxy adhesive and acrylic adhesive (i) are inferior in heat resistance of 250xc2x0 C. or more (soldering heat resistance) and moisture resistance. The adhesives comprising an alkoxide and a metal salt (ii to vii) have such a problem that an alcohol formed by a hydrolytic reaction or water formed by a dehydration reaction is gasified during curing by heating, whereby bubbles remain by bonding optical parts such as lenses, the adhesive becomes opaque, or sufficient adhesion cannot be obtained. The adhesives comprising an epoxy resin (viii, ix) are not satisfactory in terms of the heat resistance of the adhesive layers.
It is an object of the present invention to provide an adhesive composition which overcomes the above defects, has excellent moisture resistance and heat resistance, rarely generates bubbles during curing to eliminate such a defect as opaqueness caused by bubbles and can be used for the assembly and bonding of optical parts.
It is another object of the present invention to provide an optically transparent optical part which is bonded by the adhesive composition of the present invention.
Other objects and advantages of the present invention will become apparent from the following description.
According to the present invention, firstly, the above objects and advantages of the present invention are attained by an adhesive composition for bonding together optical parts, comprising:
(A) an epoxysilane represented by the following chemical formula (1) or a hydrolysis/polycondensation product thereof:
RnSiX4-nxe2x80x83xe2x80x83(1)
xe2x80x83wherein R is an organic group having an epoxy bond or organic group having no epoxy bond, X is a hydrolyzable group or atom, and n is 1 or 2, with the proviso that when n is 1, R is an organic group having an epoxy bond and when n is 2, at least one of R""s is an organic group having an epoxy bond;
(B) a bisphenol type epoxy resin having a viscosity of 2,000 to 5,000 mPaxc2x7s;
(C) a novolak type epoxy resin;
(D) a curing agent which is an amine; and
(E) at least one of water and an alcohol, wherein the amounts of the above components (A), (B), (C) and (D) are 3 to 60 wt %, 5 to 90 wt %, 5 to 35 wt % and 3 to 30 wt % based on the total weight of the components (A), (B) and (C), respectively and the amount of the above component (E) is 0 to 0.75 time the total number of mols of the hydrolyzable groups or atoms contained in the component (A).
Secondly, the above object sand advantages of the present invention are attained by an optical device comprising at least two optically transparent optical parts and an optically transparent adhesive layer which is formed by curing the above adhesive composition of the present invention to bond together these optical parts, wherein
when the refractive indices of two adjacent optical parts are represented by n1 and n2 (n1xe2x89xa7n2), the above adhesive layer between the adjacent optical parts satisfies a refractive index n3 represented by the following expression (1):
{square root over ((n1xc2x7n2))}xe2x88x92(({square root over ((n1xc2x7n2))}xe2x88x92n2)/3)xe2x88x920.05xe2x89xa6n3xe2x89xa6{square root over ((n1xc2x7n2))}+((n1xe2x88x92{square root over ((n1xc2x7n2))}/3)+0.05xe2x80x83xe2x80x83(1)
The adhesive composition used in the present invention comprises the components (A), (B), (C), (D) and (E). The component (A) is a component which strengthens bonding between the surface of an adherend and an adhesive and contributes to the improvement of moisture resistance. As shown by the above chemical formula (1), when n is 1, the compound used as the component (A) is a silane compound having one organic group with an epoxy bond and three hydrolyzable groups or atoms, or a hydrolysate or polycondensate thereof. When n is 2, a silane compound having one or two organic groups with an epoxy bond and two hydrolyzable groups or atoms or having one organic group with an epoxy bond and one organic group without an epoxy bond such as an alkyl group, aryl group or alkenyl group or a hydrolysis/polycondensation product thereof is used. An epoxysilane of the above chemical formula (1) in which n is 1, or a hydrolysis/polycondensation product thereof having excellent heat resistance and moisture resistance is obtained advantageously. Examples of the organic group having an epoxy bond include glycidoxypropyl group and 3,4-epoxycyclohexyl group. The glycidoxypropyl group and 3,4-epoxycyclohexyl group are preferably used because they are easily acquired. The hydrolyzable group is, for example, an alkoxyl group. Out of these, an alkoxyl group having 1 to 4 carbon atoms is preferred. The hydrolyzable atom is, for example, a halogen atom. Chlorine atom is preferred as the halogen atom. Preferred examples of the silane compound represented by the above chemical formula (1) include 3-glycidoxypropyl trimethoxysilane, 3-glycidoxypropyl triethoxysilane, 2-(3,4-epoxycyclohexyl)ethyl trimethoxysilane, 2-(3,4-epoxycyclohexyl)ethyl triethoxysilane, 3-glycidoxypropylmethyl dimethoxysilane, 3-glycidoxypropylmethyl diethoxylsilane, di(3-glycidoxypropyl)diethoxysilane, di(3-glycidoxypropyl)dimethoxysilane and hydrolysis/polycondensation products thereof.
The component (A) may be a silane compound represented by the above chemical formula (1) or a hydrolysis/polycondensation product thereof. When the hydrolysis/polycondensation product is used, a reaction is preferably carried out while an alcohol and water by-produced by the reaction are distilled off to ensure that the adhesive composition should not contain water and the alcohol substantially. When the component (A) is a silane compound represented by the above chemical formula (1) and the amount thereof is too large, the viscosity of the obtained adhesive composition becomes too low, resulting in deteriorated coatability. Meanwhile, when the amount is too small, the effect of improving moisture resistance cannot be obtained fully. When the component (A) is a hydrolysis/polycondensation product of the silane compound represented by the above chemical formula (1) and the amount thereof is too large, the viscosity of the composition becomes too high and the amounts of an alcohol and water by-produced by hydrolysis become relatively large, whereby the obtained cured product becomes porous or opaque disadvantageously. Meanwhile, when the amount is too small, the effect of improving moisture resistance cannot be obtained fully. Therefore, the amount of the component (A) is 3 to 60 wt %, preferably 6 to 50 wt %, more preferably 8 to 40 wt % based on 100 wt % of the total of the components (A), (B) and (C).
The bisphenol type epoxy resin as the component (B) is a component for forming the basic skeleton of an adhesive layer. By controlling the molecular weight of the bisphenol type epoxy resin, that is, the chemical structure thereof, the viscosity of the adhesive composition is set to 2,000 to 5,000 mPaxc2x7s. Within this viscosity range, the adhesive composition can be applied easily. Preferred examples of the component (B) include bisphenol A type epoxy resins, bisphenol F type epoxy resins and bisphenol S type epoxy resins. Out of these, bisphenol F type epoxy resins are preferred. When the amount of the component (B) is too large, heat resistance and moisture resistance lower and when the amount is too small, the compatibility of the component (B) with other components lowers. The amount of the component (B) is 5 to 90 wt %, preferably 20 to 80 wt %, more preferably 40 to 75 wt % based on 100 wt % of the total of the components (A), (B) and (C).
The component (C) is a component for improving the heat resistance of the adhesive layer. When the amount of the component (C) is too large, the viscosity of the adhesive composition becomes too high and when the amount is too small, the heat resistance is not improved fully. Therefore, the amount of the component (C) is 5 to 35 wt %, preferably 8 to 30 wt %, more preferably 12 to 28 wt % based on 100 wt % of the total of the components (A), (B) and (C).
The amine as the component (D) is a curing catalyst for polymerizing the epoxy bonds of the components (A), (B) and (C) and also serves as a hydrolytic catalyst for the component (A). Examples of the component (D) include primary amines, secondary amines and tertiary amines such as diethylenetriamine, triethylenetetramine, polymethylenediamine, methaphenilene diamine, methaphenilene diamine, diaminodiphenylmethane, imidazole, 2-methylimidazole and 2-ethyl-4-methylimidazole. Out of these, imidazoles, that is, imidazole, 2-methylimidazole and 2-ethyl-4-methylimidazole are preferred because they have excellent reactivity. The amount of the component (D) is 3 to 30 wt %, preferably 4 to 25 wt %, more preferably 5 to 20 wt % based on 100 wt % of the total of the components (A), (B) and (C).
Preferably, the adhesive composition of the present invention do not contain a large amount of a volatile component such as an alcohol or water. When the adhesive composition contains a volatile component, the obtained cured product may become porous or opaque. When a hydrolysis/polycondensation product of an epoxysilane is used as the component (A), it is preferred to reduce the contents of water and an alcohol in the component (A), that is, the adhesive composition as much as possible by carrying out a reaction while the by-produced alcohol and water are distilled off so as to prepare the hydrolysis/polycondensation product. When an unhydrolyzed epoxysilane is used as the component (A), water must be contained in the adhesive composition for the hydrolysis of this epoxysilane. Therefore, it is preferred to contain water in the adhesive composition in an amount of 0.5 to 0.75 time the total number of mols of the hydrolyzable groups or atoms of the epoxysilane. This water does not need to be added and water contained in the components (B) and (C) as an impurity suffices. When the hydrolysis and dehydration reaction of the epoxysilane occurs in the adhesive composition, part of water contained changes to an alcohol. Therefore, water and/or an alcohol as the component (E) is contained in the adhesive composition in an amount of 0 to 0.75 time the total number of mols of the hydrolyzable groups or atoms (the total of the number of mols of the groups and the number of mols of the atoms when both are contained) of the epoxysilane. More specifically, the total content of the alcohol and water contained in the adhesive composition is preferably maintained at 1 wt % or less, more preferably 0.1 wt % or less.
In the present invention, the contents of the epoxysilane, bisphenol type epoxy resin and novolak type epoxy resin in the adhesive composition are adjusted such that the refractive index value of the adhesive layer should approximate to the refractive index values of at least two optically transparent optical parts metioned above. More specifically, when the refractive indices of the two adjacent optical parts are represented by n1 and n2 (n1xe2x89xa7n2), the adhesive layer between the adjacent optical parts preferably has a refractive index n3 represented by the above expression (1), more preferably a refractive index n3 represented by the following expression (2).
{square root over ((n1xc2x7n2))}xe2x88x92(({square root over ((n1xc2x7n2))}xe2x88x92n2/10)xe2x88x920.01xe2x89xa6n3xe2x89xa6{square root over ((n1xc2x7n2))}+((n1xe2x88x92{square root over ((n1xc2x7n2)))}/10)+0.01 xe2x80x83xe2x80x83(2).
For example, when an optical fiber having a refractive index (n2) of 1.46 and a microlens having a refractive index (n1) of 1.59 are to be bonded together, 1.452xe2x89xa6n3xe2x89xa61.596 according to the expression (1) and 1.507xe2x89xa6n3xe2x89xa61.540 according to the expression (2). By adjusting the refractive index, an optical device having a small light propagation loss is thus obtained. As for bonding of optical fibers, lenses, filters, optical waveguides, diffraction gratings and optically active elements, optical devices having a small light propagation loss are obtained by adjusting the refractive index similarly.
A description is subsequently given of the optical parts of the present invention. The optical parts used in the present invention include optical fibers, lenses, filters, optical waveguides, diffraction gratings and optically active elements. The optical fibers include a single-mode optical fiber and multi-mode optical fiber. The lenses include a refractive index distribution lens, spherical lens, non-spherical lens and plane-convex lens. The optical filters include a narrow-band filter made from a dielectric multi-layer film, band-pass filter and polarization filter. The optical waveguides include a single-mode optical waveguide and multi-mode optical waveguide. These optical waveguides may have a Bragg diffraction grating with a periodically modulated refractive index. The materials constituting these optical parts include glass materials, plastic materials, and organic-inorganic composite materials.
The materials constituting the above optical parts preferably have a linear expansion coefficient of 1.5xc3x9710xe2x88x925/xc2x0 C. or less. When the linear expansion coefficient is larger than 1.5xc3x9710xe2x88x925/xc2x0 C., in the case of a plastic optical part having a high thermal expansion coefficient of 9 to 15xc3x9710xe2x88x925/xc2x0 C. such as polypropylene, delamination may occur between the optical part and the adhesive layer in the heating step after the application of an adhesive or the adhesive layer may crack. Ordinary inorganic glass has a linear expansion coefficient of 1.5xc3x9710xe2x88x925/xc2x0 C. or less. At least the bonding surface of an optical part is preferably made from an oxide. If the bonding surface is not made from an oxide, the adhesion strength of the adhesive layer lowers in the molding step and delamination occurs between the surface to be bonded and the adhesive layer as the case may be. Preferred examples of the material of the optical part include oxide glass such as silicate-based glass, boric acid-based glass and phosphoric acid-based glass, quartz, ceramics, epoxy resins and glass fiber reinforced polystyrene. Out of these, oxide glass and quartz are preferred because they have a refractive index of 1.40 to 1.55, high transparency and a low expansion coefficient. Although a metal is not bonded by the adhesive layer of the present invention as it is, if the surface of a metal is treated with an oxidizing agent, it may be used as a part to be bonded.
When these optical parts are assembled together, the optically transparent adhesive composition of the present invention is applied to be filled or spread between a first optical part and a second optical part and then cured to form a bonding portion having predetermined strength. As for the curing of the adhesive, an adhesive composition which cures in a few minutes can be obtained by increasing the amount of a curing agent as the component (D). By reducing the amount of the curing agent, an adhesive composition having a pot life of several hours can be obtained. The curing time can be shortened by heating as required. A reaction retardant and a curing accelerator may be added as required in an amount of 40 wt % or less, preferably 30 wt % or less based on the total amount. The curing time can be controlled freely by adding a reaction retardant or a curing accelerator. After the adhesive composition is applied, it is generally maintained at room temperature to 250xc2x0 C. for several seconds to several hours to be cured.